Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality displays in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M.times.N, actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electrical signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIGS. 1A to 1G, there are illustrated manufacturing steps involved in manufacturing an array 100 of M.times.N thin film actuated mirrors 101, wherein M and N are integers, disclosed in a commonly owned application, U.S. Ser. No. 08/598,478, entitled "METHOD FOR FORMING AN ARRAY OF THIN FILM ACTUATED MIRRORS", U.S. Pat. No. 5,677,785.
The process for the manufacture of the array 100 begins with the preparation of an active matrix 10 including a substrate 12 with an array of M.times.N connecting terminals 14 and an array of M.times.N transistors (not shown), wherein each of the connecting terminals 14 is electrically connected to a corresponding transistor in the array of transistors.
In an ensuing step, there is deposited a passivation layer 70, made of, e.g., PSG or silicon nitride, and having a thickness of 0.1-2 .mu.m, on top of the active matrix 10 by using, e.g., a CVD or a spin coating method.
Thereafter, an etchant stopping layer 80, made of a nitride, and having a thickness of 0.1-2 .mu.m, is deposited on top of the passivation layer 70 by using, e.g., a sputtering or a CVD method.
In a subsequent step, there is deposited on top of the etchant stopping layer 80 a thin film sacrificial layer 20, having a thickness of 0.1-2 .mu.m, and made of a metal, e.g., copper (Cu) or nickel (Ni), a phosphor-silicate glass (PSG) or a poly-Si. The thin film sacrificial layer 20 is deposited by using a sputtering or an evaporation method if the thin film sacrificial layer 20 is made of a metal, a chemical vapor deposition (CVD) method or a spin coating method if the thin film sacrificial layer 20 is made of a PSG, or a CVD method if the thin film sacrificial layer 20 is made of a poly-Si.
Thereafter, there is formed an array of M.times.N pairs of empty cavities (not shown) on the thin film sacrificial layer 20 by using an etching method, as shown in FIG. 1A. One of the empty cavities in each pair encompasses one of the connecting terminals 14.
Subsequently, an elastic layer 30, made of an insulating material, and having a thickness of 0.1-2 .mu.m, is deposited on top of the thin film sacrificial layer 20 including the empty cavities by using a CVD method.
In a next step, there is created an array of M.times.N contact holes 37 on the elastic layer 30 by using an etching method, wherein each of the contact holes 37 exposes one top of the connecting terminals 14 and has inner surfaces (not shown), as shown in FIG. 1B.
Then, a second thin film layer 40, made of an electrically conducting material, and having a thickness of 0.1-2 .mu.m, is deposited on top of the elastic layer 30 including the inner surfaces of each of the contact holes 37 by using a sputtering or a vacuum evaporation method.
Next, a thin film electrodisplacive layer 50, made of a piezoelectric or an electrostrictive material, and having a thickness of 0.1-2 .mu.m, is deposited on top of the second thin film layer 40 by using a CVD method, an evaporation method, a Sol-Gel method or a sputtering method. The thin film electrodisplacive layer 50 is then heat treated to allow a phase transition to take place, as shown in FIG. 1C.
In an ensuing step, a first thin film layer 60, made of an electrically conducting and light reflecting material, and having a thickness of 0.1-2 .mu.m, is deposited on top of the thin film electrodisplacive layer 50 by using a sputtering or a vacuum evaporation method, as shown in FIG. 1D.
After the above step, the first thin film 60, the thin film electrodisplacive 50, the second thin film 40 and the elastic layers 30 are, respectively, patterned, until top of the thin film sacrificial layer 20 is exposed, by using an etching method, e.g., a photolithography or a laser trimming method, thereby forming an array of M.times.N actuating structures 90, each of the actuating structures 90 having a first thin film electrode 65, a thin film electrodisplacive member 55, a second thin film electrode 45 and an elastic member 35, as shown in FIG. 1E. Each of the second thin film electrodes 45 is electrically connected to a corresponding connecting terminal 14, thereby functioning as a signal electrode in each of the actuating structures 90. Each of the first thin film electrodes 65 is electrically connected to ground, thereby functioning as a mirror as well as a common bias electrode in each of the actuating structures 90.
Since each of the thin film electrodisplacive members 55 is sufficiently thin, there is no need to pole it in case it is made of a piezoelectric material: for it can be poled with the electric signal applied during the operation of the thin film actuated mirrors 101.
The preceeding step is followed by completely covering each of the actuating structures 90 with a thin film protection layer (not shown).
The thin film sacrificial layer 20 is then removed by using an etching method. Finally, the thin film protection layer is removed, thereby forming the array 100 of M.times.N thin film actuated mirrors 101, as shown in FIG. 1F.
There are a number of problems associated with the contact hole 37 shown in a detailed view of FIG. 1G, however, one of them being the formation of cracks 57. The heat treatment followed by the rapid cooling of the thin film electrodisplacive layer 50 results in the formation of cracks 57 at a portion of the thin film electrodisplacive layer 50 deposited on top of the contact hole 37. The cracks 57 may lead to an establishment of an electrical connection between the first thin film electrode 65 which is subsequently formed on top of the thin film electrodisplacive member 55 and the second thin film electrode 45, resulting in a short-circuit therebetween. Since the first thin film electrode 65 in each of the actuating structures 90 is interconnected with other first thin film electrodes (not shown) in the same row or column in the array 100, if one of the actuating structures 90 becomes inoperable due the above reason, i.e., short-circuit, all of the other actuating structures 90 in the same row or column in the array 100 become inoperable.